Unveiling the removal mechanism of nanoplastics by hydroxyethylene diphosphonic acid modified sodium alginate...

The amplified ecological risk of nanoplastics (NPs, <100 nm) compared to microplastics (MPs, >0.1 μm) demands efficient removal technologies. This study presents a simple method for synthesizing three-dimensional membrane capsules from sodium alginate doped with hydroxyethylene diphosphonic acid (HEDP). The capsules, rich in phosphate (PO₄^3-), hydroxyl (-OH), and carbonyl (-COOH) groups, provided abundant active sites for NPs sequestration. Remarkably, they remove differently charged NPs (80 nm) through multiple synergistic mechanisms-hydrogen bonding, ligand exchange, and physical adsorption-with electrostatic attraction enhancing removal specifically for positively charged NPs. The removal process was size-selective, favoring smaller PS-NPs (80-100 nm) over larger ones (1000-10,000 nm). The co-existing pollutants slightly reduced performance; elevated humic acid (5-10 mg/L) significantly decreased NPs capture, probably due to electrostatic shielding, pore blocking and competitive adsorption. The capsules retained >80 ± 0.65% efficiency in diverse water matrices and displayed excellent reusability, maintaining ≥80.21 ± 0.69% removal after seven cycles. However, amplified salinity (1-5% NaCl) marginally decreased efficiency, probably due to Na⁺ competition. The capsules showed high biodegradability (>96.83 ± 4.84%) but limited biocompatibility, possibly from HEDP-mediated nutrient sequestration. Mechanistic findings endorsed by FTIR, XPS, EDX/S, and DFT suggested that electrostatic attraction between -NH₃⁺ (NPs) and -COO^-/-PO₄^3- (capsules), along with hydrogen bonding (-CH/-NH⋯HO), governed NPs capture. While SEM revealed NPs aggregation via hydrophobic interactions and physical adsorption. Overall, these inexpensive, biodegradable capsules are highly promising for capturing and recovering NPs from wastewater.